Generally, machines for optically selecting particulate articles such as agricultural products, wherein the selection depends upon the color of the surface of the article, are already in use. These selectors are structured so that the articles to be selected are dropped, a ray of light irradiates the article during the descent of the article, and the reflected or transmitted ray is measured to determine whether the article is to be selected. If the article is not selected, a blast of air is instantaneously applied to the article, thereby blowing off an abnormal particle in order to remove the same.
Since this selector performs the removal of particles by means of a blast of air, any fine dust which accompanies the article to be selected, such as sugar, particulate dust, and the like included in the grain, is blown up onto the machine. Although the space in which the selecting performance is executed is covered with a bulkhead so that dust does not adhere to various parts of the machine, the dust remains in the machine. The place through which the ray is transmitted is a transparent window, as the ray must irradiate the articles to be selected during their descent, and any dust that is blown up by the blast of air adheres to the window. If any dust adheres to the window, it obstructs the transmission of the ray, which interferes with the selection process. For this reason, it is necessary to provide a device for cleaning the dust which adheres to the window. Such a cleaning device has been proposed in the prior art. In the conventional cleaning device, a scraper such as used as a window wiper for an automobile is mounted to the surface of the window, and the scraper is adapted to be moved along the surface of the window at fixed intervals to eliminate any dust adhering to the window. Nevertheless, the conventional cleaning device has a defect in that the ray is interrupted when the scraper is moved. For this reason, the selecting work cannot be performed during the cleaning process, so that the selecting machine must be stopped whenever the cleaning work is performed. For this reason, the conventional selectors have an extremely low work efficiency.